1. Technical Field
This disclosure generally relates to oil and gas well drilling and the subsequent investigation of subterranean formations surrounding the well. More particularly, this disclosure relates to apparatus and methods for disengaging or “unsticking” a tool from the wall of the well.
2. Description of the Related Art
Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall.
For successful oil and gas exploration, it is necessary to have information about the subsurface formations that are penetrated by a wellbore. For example, one aspect of standard formation evaluation relates to the measurements of the formation pressure and formation fluid mobility. These measurements are essential to predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring formation and fluid properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wireline in electrical communication with a measurement tool that is suspended from a wireline in electrical communication with a control system disposed on the surface. The tool is lowered into a well so that it can measure formation properties at desired depths. A typical wireline tool may include a probe that may be pressed against the wellbore wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using the probe, a formation tester measures the pressure of the formation fluids and generates a pressure pulse, which is used to determine the formation permeability. The formation tester tool may also withdraw a sample of the formation fluid that is either subsequently transported to the surface for analysis or analyzed downhole.
In order to use any wireline tool, whether the tool be a resistivity, porosity or formation testing tool, the drill string must be removed from the well so that the tool can be lowered into the well. This is called a “trip” uphole. Further, the wireline tools must be lowered to the zone of interest, generally at or near the bottom of the hole. The combination of removing the drill string and lowering the wireline tool downhole is time-consuming and can take up to several hours, depending on the depth of the wellbore. Because of the great expense and rig time required to “trip” the drill pipe and lower the wireline tool down the wellbore, wireline tools are generally used only when the information is absolutely needed or when the drill string is tripped for another reason, such as changing the drill bit. Examples of wireline formation testers are described, for example, in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223.
To avoid or minimize the downtime associated with tripping the drill string, another technique for measuring formation properties has been developed in which tools and devices are positioned near the drill bit in a drilling system. Thus, formation measurements are made during the drilling process and the terminology generally used in the art is “MWD” (measurement-while-drilling) and “LWD” (logging-while-drilling). A variety of downhole MWD and LWD drilling tools are commercially available.
MWD typically refers to measuring the drill bit trajectory as well as wellbore temperature and pressure, while LWD refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, allows the drilling company to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. While LWD and MWD have different meanings to those of ordinary skill in the art, that distinction is not germane to this disclosure, and therefore this disclosure does not distinguish between the two terms. Furthermore, LWD and MWD are not necessarily performed while the drill bit is actually cutting through the formation. For example, LWD and MWD may occur during interruptions in the drilling process, such as when the drill bit is briefly stopped to take measurements, after which drilling resumes. Measurements taken during intermittent breaks in drilling are still considered to be made “while-drilling” because they do not require the drill string to be tripped.
Formation evaluation, whether during a wireline operation or while drilling, often requires that fluid from the formation be drawn into a downhole tool for testing and/or sampling. Various sampling devices, typically referred to as probes, are extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and to draw fluid into the downhole tool. A typical probe is a circular element extended from the downhole tool and positioned against the sidewall of the wellbore. A rubber packer at the end of the probe is used to create a seal with the wellbore sidewall. Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
In oil and gas operations, downhole tools (such as wire line tools or drill strings) are conveyed into and withdrawn from the wellbore. Occasionally, during operation, the downhole tool may become stuck in the wellbore. Tool sticking often occurs during formation evaluation procedures, such as coring or formation fluid sampling, where a piston and/or a probe are extended into contact with the mudcake lining the wellbore. Alternatively, a tool may also become stuck during delivery into or removal from the wellbore should it contact with and breach the integrity of the mudcake layer. The formation itself is typically at a relatively lower pressure, while the wellbore is at a relatively higher pressure. Consequently, it is possible for a downhole tool to dislodge a portion of the mudcake layer and expose the tool to a significant pressure differential that holds the tool against the wellbore wall. The holding force generated by the pressure differential is difficult to overcome and often may exceed the force capable of being generated by a backup piston, probe, or other extendible component of the tool. The use of pistons to dislodge a stuck tool is also unsatisfactory because the exact portion of the tool that is in contact with the wall is typically not known, and therefore several pistons spaced circumferentially about the tool must be provided in order to insure that a pushing force can be generated in the appropriate direction. Such pistons can be damaged during tool release operations, preventing their retraction and exacerbating the sticking problem. Other known methods for disengaging downhole tools, such as fishing, cable pulling, and tool pushing by tubing, are overly difficult and time consuming.